Protection of the environment and human beings has become increasingly important. Buildings, vehicles such as cars, buses and aircraft, household appliances and industrial machinery have noise producing components such as engines, motors, gears, transmissions etc. In order to protect individuals from such noise, the noise generating components and the transmission path of the noise to a human being have been investigated with the purpose of reducing the generated noise at the source and of reducing the noise transmitted from the source to human beings.
Testing of acoustic properties of noise generating and noise transmitting media such as mechanical structures and air or other fluids is an important part of the process of noise reduction. In complex structures with several noise sources, such as mentioned above, it can be complicated to identify noise sources and transmission paths and their contributions to the perceived noise.
Mathematical models and computerised methods exist for vibro-acoustic analysis of physical structures. Acoustical tools exist for simulating acoustic properties of portions of a human being, such as Mouth Simulator type 4227, Ear Simulators types 4185 and 4195, Head and Torso Simulators types 4100 and 4128, all from Brüel & Kjær Sound and Vibration Measurement A/S, Denmark. All of these are intended for use in analysing sound at different stages in its “normal” forward transmission from the source to a human being.
EP 0 015 852 discloses a three-dimensional array of microphones for measuring the total or directional acoustic power emitted by a sound source. Such an array is suitable for use with the method of the present invention.
U.S. Pat. No. 2002/0035456 discloses a method for predicting the sound pressure at a point resulting from waves generated by or scattered from a body. The method uses acoustic transfer vectors and the reciprocity principle. A purely numerical reciprocal determination of acoustic transfer vectors is disclosed through simulation of a monopole point source in the listening position, numerical determination of the response at the body surface, and hence elements in the acoustic transfer vector.
The transfer function for sound from a small omni-directional sound source to a point of measurement is often expressed as the acoustic transfer function H (or acoustic transfer impedance Zt) defined as H=p/Q, where Q is the volume velocity emitted from the sound source, and p is the sound pressure at the point of measurement resulting from the volume velocity Q generated by the sound source. In most cases the analysed mechanical and acoustic transmission media are reciprocal, which means that the acoustic transfer function is the same both for forward and reverse transmission. In other words, if the sound source and the measuring microphone are interchanged, whereby the transmission of sound through the media is reversed, and boundary conditions remain unchanged, then the acoustic transfer impedance is unaffected, i.e. the “forward” acoustic transfer impedance and the “reverse” acoustic transfer impedance are identical.
It is known to use this fact when analysing the transmission of sound, whereby a sound source is placed in a position that is normally occupied by a human being, i.e. a “listening” position, and a microphone is placed in the normal position of the sound source. This has distinct advantages when identifying sound sources and tracking the noise on its path from the source to the listening position.
When measuring the forward transmission path a Head and Torso Simulator type 4100 from Brüel & Kjær Sound and Vibration Measurement A/S can be placed in the listening position, whereby very realistic measurements of the forward transmission path can be obtained, since the influence of the head and the torso on the transfer function to the ears is taken into account. Danish patent application PA 200300589 discloses a simulator simulating acoustic properties of the head and possibly the torso of a human being. That simulator comprises a sound source for outputting sound signals through the simulated ears. Such a simulator completes the reverse measuring chain and can be placed in a position that is normally occupied by a human being, i.e. a “listening position”. By means of a pair of microphones in each simulated ear canal the output sound volume velocity can be measured. This is useful for computing the acoustical transfer function from a sound source to a listening position.
When designing e.g. vehicles such as cars, buses and aircraft the comfort of the passengers, the driver and crewmembers is of importance. Noise can seriously jeopardize not only comfort but also the health of humans. It is therefore important to reduce noise, and for effectively reducing noise it is important to identify noise sources and their individual contribution to the noise level at locations where people are present. Mechanical structures such as body and wall panels can vibrate and emit noise, and large structures can have “hot spots” that emit more noise than “cold spots”. Not all hot spots may be serious contributors to the noise level resulting at a “listening position”, and, vice versa, cold spots may contribute more seriously than expected. Such phenomena can e.g. be due to conditions in the transmission path from the source to the listening position.
The problem to be solved by the invention is to provide a method of determining, in a predefined position such as a listening position of a human being, the sound pressure resulting from sound emitted from a surface element of a sound emitting surface. In particular there is a need for identifying, among the plurality of noise sources, which can be distributed over a large area, the most significant sources and their contributions to the noise level at one or more listening positions.